It happens just often enough that the dealerships use that to deny warranty claims all the time. CP4’s eating themselves is a known problem but even if it was just a run of the mill CP4 come apart. Dealership has to be the only one to work on it for it to be under warranty. They’ll say you put DEF in the diesel tank and deny the warranty.
It would be nice if warranty would cover it, but I also think it shouldn't be used when it was his own negligent that caused the problem. But I can guarantee he won't do that again. I'm also guessing the dealership will replace everything, just because they can, he won't know what was actually damaged. Last I heard, it was going to be over 12 grand to fix it.